In filtration separations of materials, ranging from coarse particles to dissolved salts, from liquids, it has long been recognized that it is advantageous to configure the filter with pores on the feed side as small as necessary for the removal and with much larger pores over the rest of the filter thickness. Such a configuration, not only allows high production rates with minimal pressure drops, but also hinders plugging of the pores in the high-permeability substructure with extraneous materials in the feed.
With formed-in-place, or dynamically formed, membranes, there are further advantages from thin layers on the feed side. For many membrane-forming additives, there are optimal pore sizes, frequently in the order of tenths of a micrometer. Also, many applications would not require membranes if the pores on the feed side were smaller.
U.S. Pat. No. 4,888,114 teaches a method of fabricating a porous metal support with a fired layer of metal-oxide particulates on the surface of one side of the porous support. The result is the reduction of pore sizes at the surface from several micrometers to tenths of a micrometer. The sizes of the pores of this altered-substrate are in a much more favorable range for formation of membranes in place and are far better for certain biological and biochemical separations carried out on porous tubes without membranes. Altered substrates circumvented the disadvantages of modifying pore sizes by what might be called a formed-in-place approach, that is by circulating fine particulates through the tubes before or during the process of membrane formation. The main disadvantages overcome were the difficulties of pumping particulates, problems in getting a uniform coating of them, and instability of the coatings in some cases.
Although altered substrates are a tremendous advance over prior technology, simple methods allowing further reduction and control of pore sizes would be advantageous both for in-place formation of membranes and for separations using the bare tubes. The process of this invention provides a method for doing this.
Of interest in the present case are sol-gel processes that are widely used in formations of ceramics for various applications; such as for tubes, catalysts, and nuclear fuel elements. The objective of such processes is to form the particles that comprise ceramics. Hydrolysis and polycondensation reactions of homogeneous solutions of metal alkoxides eventually yield polymeric networks of metal oxides. Many reviews can be found of this field; a recent one is by Burggraaf, et al., Solid State Ionics 32/33, 771-782 (1989).
J. P. Clement, H. J. Rack, K. T. Wu, and H. G. Spencer, Materials and Manufacturing Processes, 5, 17-33 (1990) modified the sol-gel technology to a process for forming controlled thickness and crystalline films on structural substrates such as carbon fibers to increase wettability of the substrate by molten metal and decrease adverse interaction of the substrate with the metal. Hydrolysis is carried out in two steps, by providing a limited quantity of water for the first step. The hydrolysis is completed and the coating stabilized by the subsequent exposure to water vapor and thermal treatment. This work is covered in co-pending U.S. patent application Ser. No. 07/536,540, incorporated herein by reference, on which the present inventor is also an inventor.
Clement et al. teach dip-coating to form the initial film, and curing by heating in a carbon monoxide atmosphere at 700.degree. C. Time for performing the steps was not important.
The time allowed here is important in the control of the product.